The present invention relates to an impedance means and to a method for adjusting the impedance of the impedance means. The invention also relates to electronic circuits including such an impedance means. The invention also relates to an integrated circuit chip.
A circuit component is frequently described as having an impedance Z with a resistive part and a reactive part. In other words a component has a resistance R and a reactance X. The reactance may be capacitive or inductive. The reactance X may include a capacitive component as well as an inductive component, rendering the impedance of the component inductive and capacitive as well as resistive.
Resonator circuits are components used in electronic circuits such as filters and oscillators. U.S. Pat. No. 4,338,582 discloses a resonant circuit comprising an inductor, a varactor and a resistor coupled with a Field Effect Transistor. The Field Effect Transistor is arranged to exhibit negative resistance, so as to compensate for a large resistive component exhibited by the varactor.
The publication "Varactor-Tunable, High-Q Microwave Filter" by A. Presser (RCA Review, Vol 42, pp 691-705, 1981) also concludes that varactor losses can be compensated by the negative resistance of an active element. Presser discloses a Field Effect Transistor which is connected in a feedback configuration so as to provide the negative resistance. The disclosed Field Effect Transistor is connected in series with a varactor with resistive losses.
The article "SiIc-Compatible Inductors and LC Passive Filters" (IEEE Journal of Solid-State Circuits, Vol. 25, No. 4, 1990) by N. M. Nguyen and R. G. Meyer, discloses a square spiral inductor which is fabricated of aluminium on a silicon substrate. The disclosed inductor having an inductance of 9.7 nH also had a series resistance of 15.4 .OMEGA. and a maximum Q-value below 4 at 0.9 GHz. The article also discloses a low pass filter including two inductors with mutual inductance between them. Although a Q-value of about 3 is usable, the performance of the disclosed inductors is still limited by metal resistance.
EP, A2, 0 356 109 describes a low frequency inductance transformer having a primary winding and a secondary winding. A sensor transformer senses the current in the primary winding of the inductance transformer and an operational amplifier, responsive to the sensed current, is described for controlling the current in the secondary winding to increase the inductance effect of the primary winding.